U.S. Pat. No. 420,384 and U.S. Pat. No. 6,551,565 discloses processes to separate catalyst from FCC flue gasses.
A paper titled “FCCU Particulate Emissions Control with a Shell Third Stage Separator—A Case Study” as presented by Edwin H. Weaver, of Belco Technologies Corporation at the 2002 NPRA, New Orleans describes how catalyst fines are separated from the flue gas of a FCC regenerator. According this paper Third Stage Separators (TSS) have been utilized for many years to separate catalyst fines from the regenerator flue gas in order to protect turbo expanders installed downstream said separator. Typical third stage separators are for example described in Hydrocarbon Processing, January 1985, 51-54. For many years, the TSS has been used as an effective device to remove catalyst fines from the FCCU regenerator flue gas in order to provide protection for a turbo expander. In this service, it was critical that the TSS removes a sufficient quantity of the catalyst fines so that the remaining catalyst fines in the flue gas would not damage the turbo expander.
In a TSS solids are separated from the majority of the gas flow by means of a plurality of swirl tube separators operating in parallel within a common vessel. The solids content in the gas is lowered to for example 43 mg/Nm3. The separated solids accumulate in the lower part of the vessel and are discharged from the vessel together with a small quantity of the flue gas. This solids rich flow is also referred to as the TSS underflow. This TSS underflow is then routed to a fourth stage separator. In the fourth stage separator solids are typically processed by a so-called 4th stage cyclones, or more recently, removed entirely by passing the gas through a cyclone, a ceramic or composite filter. In the latter case, the gas containing a small amount of particulates, for example 2 mg/Nm3, is then routed back to the main flue gas flow downstream the TSS.
A disadvantage of the process as described in the above referred to conference paper is that an absolute filter has to be used to minimize the particulate emission from the process. These filters have the advantage that almost all solids can be separated from the underflow gasses. However these filters have the disadvantage that in addition to their high cost, the (ceramic) filter candles may break resulting in solids ending up in the flue gas. Metal filter candles have the disadvantage that they cannot be applied at the normal operating temperature of the process and gas cooling needs to be applied.
The aim of the present invention is to provide a process, which results in a flue gas having a low solids content wherein no absolute filters have to be used.